I agree. These days we used to report such things on https://webcompat.com/ . Try to report it there, they should know what to do.

But only if the actual health provider blocks your access, or if it's genuinely broken.

This platform lists Firefox both on desktop and mobile as supported, so should be working flawlessly.

This is complicated. This leaflet looks like coming from the health provider, not the actual platform itself… because it's definitely a couple of years old compared to the site mentioned in the text, so it's not current. It kinda feels like something written up by the health provider some time ago, and reused since then, so it might be a little oversimplified for them to not have to spend too much time supporting their local patients.

The actual platform https://en.wikipedia.org/wiki/Doxy.me is pretty well-written, and does not make too much claims about support, e.g.: https://help.doxy.me/en/articles/3921965-for-patients-technical-guide — "Use a Supported Browser: Ensure you are using the latest version of a modern browser like Chrome, Firefox, or Safari, as they offer the best support for WebRTC." — so these guys are trying to make the platform as accessible as possible.

But in general yes, a lot of the big telehealth providers have rather bad interoperability, and basically only write their apps for a single engine. And once written, it usually runs years and years without any updates. They minimize the support footprint by removing any engine they don't necessarily have to test with — and given patients end up with no choice than to accept the telemedicine requirements if they need the service, the giant corps have no need to change that behavior:/